The Suburban tank mentioned is goes back to at least '60. And they aren't available new.
How are you at welding/fab? Here's why (sorry, can't think of a succinct way to write this).
Imagine the gas tank is a rectangular box. Divide that box in half the long way (side-to-side). The top half is still squared off, but the bottom half has a half-circular notch cut into it for half the spare tire to nest into when the tire is mounted under the floor.
Now, '55-'57 Chevy station wagons have a similar tank, but on those, the spare tire is in a compartment recessed into the cargo area floor. This puts the semi-circular tire nesting area on the top half of the station wagon tank.
You could flip a station wagon tank upside down for use in a Suburban, but you'd need to move the fill tube mount, fuel sender mount, fuel gauge mount and drain plug to the other side of the tank. It could be done, and if someone is good at fabrication, it could be done fairly easily.
Both tanks mount forward of the spare tire, between the tire and the rear axle.
-Brad
